Monoclonal antibodies (mAb) are fundamental tools for the advancement of biomedical research and the understanding, detection and treatment of cancer. The Fox Chase Cancer Center (FCCC) Hybridoma Facility has the expertise and resources both for the custom generation of hybridomas and for the production and purification of mAb. The Facility enables a diverse group of scientists, especially those unfamiliar with the specialized methods required to produce hybridomas, to obtain valuable antibody reagents efficiently and economically. As a reflection of this assistance, eight laboratories in the Center have become new users of the Facility since 1999. In addition, the number of laboratories storing cells in the liquid nitrogen freezers maintained by the Facility expanded from 23 to 26, thus reducing the duplication of these freezers in individual laboratories. Currently, investigators outside the Immunobiology Program comprise the major user group in the Center. Overall, 26 investigators with peer-reviewed funding (25% increase since 1999) participating in 8 of the 11 Research Programs, representing all three Divisions of the Center, use the Facility. In 2003, peer-reviewed usage of the Facility was 98%. Since 1999, new Facility services include in vitro production of high-titer mAb (a goal of the last review), quantitation of in vitro murine mAb production by ELISA and monitoring of in vitro production of rabbit, rat and hamster mAb by slide-electrophoresis, purification of mAb, polyacrylamide gel electrophoresis to determine the purity of purified mAb, labeling mAb with fluorescent dyes and production of recombinant IL-6. Demand since the last review has also grown for older services including hybricloma cloning (32% increase) and mAb isotyping (30% increase). Services provided are at near capacity for the Facility staff (1.5 FTE). A future goal of the Facility is to improve the efficiency and cost-effectiveness of producing rabbit hybridomas, as rabbits usually produce high affinity antibodies and are the first choice among many investigators for raising antibodies. The Facility will also generate and select a panel of rat and rabbit monoclonal antibodies that recognize epitope tags that are often engineered into recombinant proteins. Such tag-specific mAb could be used for detection and purification of tagged proteins when a protein-specific mAb is not yet available and would also be useful for sandwich-ELISA.